Appendix II. 819 the height of the tool point should be exactly level with the centre line of the work; for the section A B of a cone, Fig. 788, through the axis, is a triangle, for which a straight-line feed would be suitable, but the section at CD, below the axis, is a hyperbola, and a straight-line feed would simply produce a reversed hyperbola instead of a true cone. P. 249. Tooling Circular Arcs.—The methods of tooling arcs of large radius may thus be classified :— 1. Milling or slotting, with hand feed. 2. Milling in a profiling machine having a curved copy. 3. Special milling apparatus, p. 752, using a property of the circle. 4. Planing on a pivoted table controlled by a rod equal in length to arc radius. 5. Turning on a large face plate in a vertical lathe, whose short mandrel is sunk in the ground. Numbers i, 2, and 3 have already been described. No. 5 is practised at Woolwich on racers or roller paths of large radius. But as large arcs are not often required, it is better to fit up a planing machine in the manner shewn at Fig. 789, where two 789. PlarvLrut ones positions are given. Taking position L, let a line A B be drawn immediately under the tool point, and let a stud B be fixed to the table. Next, let a triangular frame A CD, one side of which is the small table c D, be pivoted on stud B, and further pinned at A; the hole in CD being slotted to permit the planing table to travel, The work to be planed is now fastened to c D, and the planing commences; then, by referring to position II., it will be found that the curve c D, struck from A, will always lie under the tool point whatever the table position.